This invention relates to electrochemical cells and batteries, and more particularly, to such cells and batteries having lithium-based active material.
Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells have included an anode (negative electrode), a cathode (positive electrode), and an electrolyte interposed between electrically insulated, spaced apart positive and negative electrodes. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically nonaqueous (aprotic) organic solvents. By convention, during discharge of the cell, the negative electrode of the cell is defined as the anode. During use of the cell, lithium ions (Li+) are transferred to the negative electrode on charging. During discharge, lithium ions (Li+) are transferred from the negative electrode (anode) to the positive electrode (cathode). Upon subsequent charge and discharge, the lithium ions (Li+) are transported between the electrodes. Cells having metallic lithium anode and metal chalcogenide cathode are charged in an initial condition. During discharge, lithium ions from the metallic anode pass through the liquid electrolyte to the electrochemically active material of the cathode whereupon electrical energy is released. During charging, the flow of lithium ions is reversed and they are transferred from the positive electrode active material through the ion conducting electrolyte and then back to the lithium negative electrode.
The lithium metal anode has been replaced with a carbon anode, that is, a carbonaceous material, such as non-graphitic amorphous coke, graphitic carbon, or graphites, which are intercalation compounds. This presents a relatively advantageous rechargeable lithium battery in which lithium metal is replaced with a material capable of reversibly intercalating lithium ions, thereby providing the xe2x80x9crocking chairxe2x80x9d battery in which lithium ions xe2x80x9crockxe2x80x9d between the intercalation electrodes during the charging/discharging/recharging cycles. Such lithium metal free cells may thus be viewed as comprising two lithium ion intercalating (absorbing) electrode xe2x80x9cspongesxe2x80x9d separated by a lithium ion conducting electrolyte usually comprising a lithium salt dissolved in nonaqueous solvent or a mixture of such solvents. Numerous such electrolytes, salts, and solvents are known in the art.
Preferred positive electrode active materials include LiCoO2, LiMn2O4, and LiNiO2. The cobalt compounds are relatively expensive and the nickel compounds are difficult to synthesize. A relatively economical positive electrode is LiMn2O4, prepared by reacting generally stoichiometric quantities of a lithium-containing compound and a manganese containing compound. The lithium cobalt oxide (LiCoO2), the lithium manganese oxide (LiMn2O4), and the lithium nickel oxide (LiNiO2) all have a common disadvantage in that the charge capacity of a cell comprising such cathodes suffers a significant loss in capacity. That is, the initial capacity available (amp hours/gram) from LiMn2O4, LiNiO2, and LiCoO2 is less than the theoretical capacity because less than 1 atomic unit of lithium engages in the electrochemical reaction. Such an initial capacity value is significantly diminished during the first cycle operation and such capacity further diminishes on every successive cycle of operation. The specific capacity for LiMn2O4 is at best 148 milliamp hours per gram. As described by those skilled in the field, the observed reversible capacity is on the order of 60% of the aforesaid value. Obviously, there is a tremendous difference between the theoretical capacity (assuming all lithium is extracted from LiMn2O4) and the actual capacity when much less than one atomic unit of lithium is extracted as observed during operation of a cell. For the metal oxides listed above, only about 0.5 atomic units of lithium is reversibly cycled during cell operation. Thus, the presently known and commonly used, alkali transition metal oxide compounds suffer from relatively low capacity. Therefore, there remains the difficulty of obtaining a lithium-containing chalcogenide electrode material having acceptable capacity without disadvantage of significant capacity fading (loss) when used in a cell.
Capacity fading is well known and is calculated according to the equation given below. The equation is used to calculate the first cycle capacity loss. This same equation is also used to calculate subsequent progressive capacity loss during subsequent cycling relative back to the first cycle capacity charge reference.
xe2x80x83((FC charge capacity)xe2x88x92(FC discharge capacity))xc3x97100% FC charge capacity
In view of the present state of the art, there remains the difficulty of obtaining lithium manganese oxide based electrode materials having the attractive capacity of the basic spinel LixMn2O4 intercalation compound, but without its disadvantage of significant capacity loss on progressive cycling.
In one embodiment, the invention provides a novel composition which is stabilized against decomposition when used as an active material for an electrochemical cell. Problems observed with degradation and decomposition of active material have been described in co-pending applications Ser. No. 09/307,335, filed May 7, 1999, and in then co-pending PCT/US97/22525, filed Nov. 21, 1997, and in then co-pending U.S. Ser. No. 08/762,081, filed Dec. 9, 1996, now U.S. Pat. No. 5,869,207, in the names of J. Barker, Y. Saidi, and C. S. Kelley and assigned to the assignee of the present invention. The active material of the present invention comprises particles of spinel lithium manganese oxide (LMO) enriched with lithium by a decomposition product of a lithium-containing compound. This lithium-containing compound meets three criteria: (1) it is lithium-containing; (2) decomposable at low enough temperature to release Li and not cause oxygen deficiency from the LMO; and (3) does not contain potentially harmful contaminants to the LMO performance (e.g., transition metals). Desirable compounds are, for example, lithium acetate (LiC2H3O2), lithium oxalate (Li2C2O4), lithium formate (LiOOCH), and lithium tartrate (Li2C4H4O6). The most preferred compound is lithium hydroxide. The decomposition product forms a part of each of the LMO particles. The spinel LMO product formed by the decomposition of lithium containing compound in the presence of the LMO is characterized by a reduced surface area and increased capacity retention (reduced capacity fading) as compared to the initial, non-treated, non-enriched spinel.
In one aspect, the spinel LMO product is a reaction product of the LMO particles and the lithium containing compound. The lithium-rich spinel so prepared is represented by the formula Li1+yMn2xe2x88x92yO4, where y is greater than zero and less than or equal to 0.20. Preferably, the lithium-enriched spinel LMO product has y greater than or equal to 0.08. The character of the product is further defined below. This lithium-rich spinel product is preferably prepared from an LMO starting material of the formula Li1+xMn2xe2x88x92xO4, where in the starting material has a value of x greater than zero and less than or equal to 0.08. Preferably the starting material before enrichment has a value of x greater than 0.05. The lithium-rich spinel product Li1+yMn2xe2x88x92yO4, has a lithium content greater than that of the Li1+xMn2xe2x88x92xO4 starting material.
Obviously, if all the lithium containing compound is decomposed or reacted, then the lithium-enriched spinel is produced. If some of the lithium containing compound remains unreacted or not decomposed, then presumably it is dispersed or adhered to the surface of the spinel particles. In the preferred embodiment, essentially all of the lithium containing compound is decomposed, and the lithium forms a part of the LMO product. The preferred embodiment will be described with reference to the preferred lithium hydroxide compound.
In one embodiment, the invention provides a method of treating spinel lithium manganese oxide particles which first comprises forming a mixture of the lithium manganese oxide particles and lithium hydroxide. Next, the mixture is heated for a time and in a temperature sufficient to decompose at least a portion of, and preferably all of, the lithium hydroxide in the presence of the lithium manganese oxide. Depending on the temperature selected, a portion of the lithium hydroxide is decomposed or reacted.
The result is a treated spinel lithium manganese oxide Li1+yMn2xe2x88x92yO4 characterized by reduced surface area and increased lithium content as compared to an untreated spinel lithium manganese oxide Li1+xMn2xe2x88x92xO4 where y is greater than x.
In one aspect, the aforesaid heating to decompose the lithium hydroxide is conducted in a static (i.e., non-flowing) air atmosphere or in a flowing air atmosphere. In another alternative, it is conducted in inert (e.g., nitrogen) atmosphere since the LMO would not lose oxygen at the temperature required for the Li incorporation reaction (e.g., 400xc2x0 C.). The heating is conducted for a time of up to about 5 hours and the amount of lithium hydroxide contained in the mixture, is up to about 10%, and desirably up to about 5% by weight of the total mixture. This weight percent is based on the lithium hydroxide being first air dried since it is somewhat hydroscopic and it is preferred to evaporate any water. Preferably, the amount of anhydrous lithium hydroxide in the mixture is on the order of 1.2 weight percent with the balance being the LMO. The process of, the invention can utilize either anhydrous LiOH, or LiOH H2O (monohydrate). The monohydrate is the stable hydrate formed. If using the monohydrate, obviously a change has to be made to the weight of reactant used to compensate for the H2O content.
In one alternative, essentially all of the lithium hydroxide is decomposed or reacted with the lithium manganese oxide. Then, the lithium-enriched spinel manganese oxide particles are combined with lithium carbonate particles to form a cathode mixture having further improved performance even beyond that of the lithium-enriched spinel. It is preferred that the heat-treated lithium-enriched spinel in particle form is mixed with lithium carbonate in particle form. This preferred particle mixture is used to form an electrode. The electrode comprises the particle mixture, a binder, and optionally conductive materials such as carbon powder.
Objects, features, and advantages of the invention include an improved electrochemical cell or battery based on lithium which has attractive charging and discharging characteristics; a good discharge capacity; and which maintains its integrity over many cycles as compared to presently used cells.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.